A dictionary {parameter_name:boolean} of parameters to not be
varied during fitting. True means the parameter is held fixed.
Alternatively the fixed
property of a parameter may be used.

tied : dict, optional

A dictionary {parameter_name:callable} of parameters which are
linked to some other parameter. The dictionary values are callables
providing the linking relationship. Alternatively the
tied property of a parameter
may be used.

bounds : dict, optional

A dictionary {parameter_name:value} of lower and upper bounds of
parameters. Keys are parameter names. Values are a list or a tuple
of length 2 giving the desired range for the parameter.
Alternatively, the
min and
max properties of a parameter
may be used.

eqcons : list, optional

A list of functions of length n such that eqcons[j](x0,*args)==0.0 in a successfully optimized problem.

ineqcons : list, optional

A list of functions of length n such that ieqcons[j](x0,*args)>=0.0 is a successfully optimized problem.

This property is used to indicate what units or sets of units the
evaluate method expects, and returns a dictionary mapping inputs to
units (or None if any units are accepted).

Model sub-classes can also use function annotations in evaluate to
indicate valid input units, in which case this property should
not be overriden since it will return the input units based on the
annotations.